2-stroke vs 4-stroke

[Elton]

In simple terms, what are the performance differences between 2-stroke and 4-stroke outboards? i.e. power output and fuel consumption ratios between a similar sized engine of each?

[SandTiger]

See http://www.smalloutboards.com/4Stroke.htm & http://www.clarkslandingmarina.com/4STKOR2STK.ivnu

 

Hopefully that will be of help

[slackline]

In simple terms, what are the performance differences between 2-stroke and 4-stroke outboards? i.e. power output and fuel consumption ratios between a similar sized engine of each?

In simple terms the 2stroke design needs to be reving at a maximum before it becomes most eficent and achieve full power. where as a 4 stroke has a much better power band and much higher torque capability per cc of engine volume this has 2 advantages the power is there from low revs and generally you can swing a larger of greater pitch on the prop (when it get to prop calculations you had best talk to your local witch doctor for this is surely black magic)

[ColinW]

Many years ago I had a Suzuki GS125, a four stroke motorbike. That would do about 100mpg whereas the 100cc two strokes did about 50mpg. The two strokes were about 10% more powerful even though they were smaller.

I used to use a Yamaha 5hp and later a Johnson 4hp, both two strokes, on my boat. If I did a lot of trolling at low speed I could guarantee having to stop and change a spark plug after a couple of hours, especially tthe lower one on the Johnson, even on low oil mixes (I took several plugs to save cleaning them out on the lake). I've been told four strokes will troll all day without oiling up. If you just use the boat to get from A to B at high throttle then oiling up shouldn't be a problem.

 

One thing has always puzzled me. When I started either of my outboards I had to close the choke immediately or they would die, yet four stroke motorbike engines like to be warmed up. Why is this?

[Newt]

These days you also have the option of the hybrid versions like the Optimax outboards Mecury makes and similar ones from Johnson (and maybe others). Performance & weight similar to a standard 2 stroke but behavior more like a 4 stroke including the much lower emissions and much better fuel efficiency. I'm running one of the 1.5 L outboards now and I love the thing.

 

http://northamerica.mercurymarine.com/engi...boards/optimax/

[Ken Davison South Wales]

I used to use a Yamaha 5hp and later a Johnson 4hp, both two strokes, on my boat. If I did a lot of trolling at low speed I could guarantee having to stop and change a spark plug after a couple of hours, especially tthe lower one on the Johnson, even on low oil mixes (I took several plugs to save cleaning them out on the lake). I've been told four strokes will troll all day without oiling up. If you just use the boat to get from A to B at high throttle then oiling up shouldn't be a problem.

 

I had several Seagull outboard engines and did not suffer the oiling up problem, used to go from Poole Harbour out into Bournemouth Bay and fish all day moving quite a lot whilst drifting for plaice over the Durly Rocks.

 

Standard procedure on end of day was to run engine in 45 gal. drum of clean water, remove spark plug and clean ready for next outing.

 

Had a Seagull 40, Century and Century Plus long shaft all performed very well.

[Seahog]

I've had New four and two stroke engine! ...

 

Four stroke engines every time .... and Japanese manufactured for quality and reliability.

[doshkadog]

Used a seagul and found it fiddly and troublesome (it was 40 yars old) just picked up a yamaha 2.5 HP motor and its a totally different beast, quieter, more fuel efficient, easier to start, no two stroke mixing required, but it is heavier.

 

From a theoretical point of view the two stokes will develop more power per CC than a four stoke as they have a compresion stoke each revolution where as a fourstorke only has a compresion stroke every two cycles. Therefore to equal the horse power you need a larger CC in a four stroke.

[WALCOTT]

Go for a 4 Stroke,they are far better now than the old mixing job with oil..I think over the years I have had all the models from Seagull..and alot of fast rev engines on 2 stroke ..but the new 4 ones are alot better..we had a air cooled outboard ,for launching off the beach,netting in winter it was the ****s..think it was a tomatsu or something,,not seen one for years but you could have it on tick over the prop was still and away you went...looked at diesel ones aswell but the price..think a 10hp was about £3000

[darlick]

Go for a 4 Stroke,they are far better now than the old mixing job with oil..I think over the years I have had all the models from Seagull..and alot of fast rev engines on 2 stroke ..but the new 4 ones are alot better..we had a air cooled outboard ,for launching off the beach,netting in winter it was the ****s..think it was a tomatsu or something,,not seen one for years but you could have it on tick over the prop was still and away you went...looked at diesel ones aswell but the price..think a 10hp was about £3000

I have a new jhonson 9.9 hp four stroke and its the best motor i have had, no messing about mixing petrol,cleaner running much better petrol consumption used to use twenty litre of petrol a day with 9.9 hp two stroke now its down to six litre.

Only thing i would do next time is add electric start,

Four stroke for me any day.